Increased pituitary LHRH responsiveness and divergence of gonadotropin secretion are two proestrus associated events in the rat estrous cycle. Although unknown mechanisms are responsible for the two phenomena, the proestrous gonadal steroid milieu may be involved. LHRH will induce the release of both LH and FSH in the presence of Ca++ and LHRH action is thought to be modulable by gonadal steroids. Since the hypothalamus releases LHRH in pulsatile fashion and no FSHRH has been unequivocally demonstrated, it is hypothesized that divergence may be induced by direct pituitary steroid effects in combination with variations in pulsatile LHRH; the central intent of this proposal is to test this hypothesis utilizing superfused pituitary cells. The comparative LH and FSH LHRH responsiveness of pituitary cells from each day of the rat cycle will be determined; circulating levels of gonadotropins and gonadal steroids will be determined in the donor animals. These cells will have experienced in vivo steroid priming and no supplemental steroids will be applied. Those estrous stages evidencing divergent trends in LH and FSH release will be exposed to additional LHRH pulse patterns to determine if divergence is modulable solely by circulating gonadal steroids or if it is also modulable by superimposed pulsatile LHRH patterns. Response patterns will be compared utilizing the parameters of maximum pulse amplitude, total LHRH induced hormone release and the FSH/LH ratio generated by the various pulse regimens employed. LHRH responsive cells will be fractionated on reorienting gradients, subsequently cultured and exposed to LHRH regimens chosen from initial studies to learn if altered responsiveness and divergence arise from different gonadotroph populations. The role of E2 and P4 in heightened responsiveness and divergence will also be investigated by exposing diestrous 1 pituitary cells to gonadal steroid levels observed in circulation simultaneously with estrous stages of marked divergence. It is the goal of this proposal to elucidate the effects of LHRH pulse frequency and amplitude as well as steroid effects on gonadotropin release and facilitate the more effective use of LHRH in fertility control and in the treatment of certain clinically recognized reproductive disorders.